The present invention relates to electroplating apparatus and processes, and in particular to electroplating techniques used for forming ferromagnetic films on magnetic tape heads and magnetoresistive sensors.
Electroplating processes are used to form a variety of coatings including protective and corrosion resistant coatings, jewelry and decorative coatings, and magnetic or electrical coatings. Electroplating is typically performed in an electrolytic cell comprising (i) an anode (positive electrode) having a positive voltage applied thereto, which can have the same chemical composition as the material being plated; and (ii) a cathode (negative electrode) having a negative voltage applied thereto, and which is typically the object to be electroplated (usually a metal, ceramic, or polymer structure). An electroplating solution or bath encompassing the anode and cathode contains plating ions of the metals being plated in a suitable solvent, e.g., Cu.sup.2+ and Ni.sup.2+ in water. When a potential is applied to the electrodes, the cations (positive ions) in the electroplating solution travel to the cathode, and the anions (negative ions) travel to the anode. In the electroplating process, metallic plating cations deposit on the cathode to form a thin layer of metal plating (such as chromium, copper, nickel, iron, silver, and/or cobalt) when an electric current is passed through the aqueous salt solution.
Electrodeposition from aqueous baths of the salts of ferromagnetic metals is commonly used to manufacture magnetic alloy films that form magnetic poles and shields, of magnetic sensors and heads on disk drives and tape drives. These magnetic devices use magnetically anisotropic films that are formed by electroplating a magnetic alloy under the influence of an orienting magnetic field. The electroplated film exhibits magnetic anisotropy in the plane of the film, the direction of the orienting field applied during deposition becoming the longitudinal, preferred, or easy axis of magnetization in the plated coating; and the orthogonal direction becoming the transverse or hard axis of magnetization. It is desirable for the electrodeposited magnetic film to have a large high frequency magnetic permeability. Such magnetic films have a magnetic anisotropy; directional fields are typically used to switch the device from one direction to another. The reversal of a magnetically anisotropic film takes place by rotational switching techniques, as opposed to domain wall switching which is much slower and more noisy. It is also desirable for the magnetic film to have a high permeability to support a large magnetic flux at relatively high frequencies at which the film switches magnetization states by rotation, typically at frequencies from about 0.1 MHZ to about 100 MHZ. High permeability can be maintained at high frequencies with the use of magnetic material having a high resistivity, which reduces the power loss arising from eddy currents.
Typical magnetic thin film heads are made using layers of magnetic materials, such as alloys of nickel, iron, and/or cobalt, for example PERMALLOY comprising Ni.sub.82 Fe.sub.18 which has a relatively high saturation magnetization of about 10,000 Gauss (the saturation magnetization (M.sub.s) is the magnetization provided when all the domains in the magnetic material are aligned) and a low resistivity of about 20 .mu..OMEGA.-cm. Modern magnetic materials used to fabricate thin films include for example, CoNiFeX, NiFeX, and CoFeX alloys, where X is an element that increases the resistivity while not substantially lowering the magnetic moment or degrading magnetic anisotropy. Examples of metals which can be used as X include Cr, Mo, V, Cu, Rh, and Sn, and non-metals include P and B. These elements increase the electrical resistivity of the ferromagnetic material and improve its permeability at high frequencies, to enhance the performance of the magnetic thin film head. Elements such as P and B are particularly desirable because they form glassy or microcrystalline alloys that have high electrical resistance and soft magnetic properties.
However, problems arise in the electroplating of complex alloys, such as, CoFeX, NiFeX, CoNiX, or CoNiFeX alloys, because it is difficult to form a stable electroplating solution that contains the desired combinations of elements, particularly for X elements such as P, B or Mo; and other elements such as iron. For example, such X elements cannot be added to the solution in the form of anions in their highest stable oxidation state because the simple ionic compounds of such elements are not stable. Thus, these elements are typically added to the electroplating solution in the form of oxo-acids or the salts of oxo-acids, such as sodium molybdate for Mo additions or sodium hypophosphite for phosphorus additions. These compounds, such as HPO.sub.3.sup.-2 are unstable relative to more stable ionic species such as PO.sub.4.sup.-2. Such unstable ions (e.g., HPO.sub.3.sup.-3) diffuse to and oxidize at the anode, often irreversibly, to become either unavailable for deposition reactions at the cathode, or to form species that inhibit other necessary deposition reactions. The thermodynamics of an exemplary electroplating process is summarized in Pourbaix (electrochemical potential vs. pH) diagrams of the ionic species, as shown in FIG. 1, which is an overlay of the Pourbaix diagram for cobalt and phosphorus ions, both of which are used in many electroplating processes. The Pourbaix diagram shows the stability of the various cobalt and phosphorous containing ions for increasing pH of the solution (x-axis) and electrochemical potential applied to the substrate to be electroplated (y-axis). The graph demonstrates that at a pH of 2 to 3, which is the typical pH of the electroplating solutions, both sodium orthophosphite and sodium hypophosphite are relatively thermodynamically unstable at the electrochemical potentials needed to dissolve cobalt from a cobalt containing anode, namely at potentials greater than -0.4 volts versus the conventional hydrogen cell. At these conditions, the orthophosphite and hypophosphite anions diffuse across the solution to contact the anode and become oxidized to the phosphate anion (PO.sub.4.sup.-3). Because the phosphate anions are too thermodynamically stable to be reduced at the cathode, the phosphate anions reduce the current efficiency of the cell. Also, the phosphate anions often combine with other metal ions to precipitate out as insoluble phosphate compounds in the electroplating solution. Thus it is desirable to reduce or prevent the unstable oxidizable anions from traveling to and oxidizing at the anode.
The electroplating cell can contain unstable cation species that are also easily oxidized at the anode; for example, ferrous cations (Fe.sup.+2) can become oxidized to ferric cations (Fe.sup.+3) at the anode. This is undesirable, because ferric ions often precipitate out of solution. Yet another problem that arises when incorporating easily oxidizable species in a plating bath is that these species oxidize on contact with oxygen in ambient air surrounding the electroplating solution. However, the oxidizable species are often desirable at the cathode to allow deposition of such species on the substrate being electroplated. For example, ferrous cations (Fe.sup.+2) are needed to deposit CoFeP alloys that have highly desirable magnetic properties.
Another problem arises because the fundamental nature of electrodeposition processes requires that a reducing potential be present at the cathode and an oxidizing potential at the anode. Thus all electrodeposition processes which rely on the presence of a thermodynamically unstable species potentially suffer from undesirable oxidation or reduction of these species at the anode or cathode. The oxidized ionic species are undesirable because they remain in the electroplating bath as an additional non-reactive component that competes in the electromigration process, as a "spoiler" which impairs the cathodic electrodeposition process, or as a precipitated material. In the former case, additional quantities of the thermodynamically unstable ionic species must be continuously added to the electroplating solution; and in the latter case, the entire electroplating solution must be purified or rejected. Oxidization of thermodynamically unstable species in the cell often results in poor electroplating performance, non-uniform deposition, variable electrical and magnetic properties in the plated coating, and shortened use lifetime for the electroplating solution bath.
Thus it is desirable to have an electroplating apparatus and method that reduces or altogether prevents oxidation of thermodynamically unstable ionic species in the electroplating solution of the apparatus. It is further desirable to have an electroplating apparatus and method that can be used to uniformly deposit complex magnetic alloys onto substrates, and that can be easily adapted to deposit different alloy compositions.